PTFE – Surface Preparation
Surface preparation is the essential first stage treatment of a substrate before the application of any PTFE coating. The performance of the coating is significantly influenced by its ability to adhere properly to the substrate material. It is generally well established that correct surface preparation is the most important factor affecting the total success of surface treatment. The presence of even small amounts of surface contaminants, oil, grease, oxides etc. can physically impair and reduce coating adhesion to the substrate. In summary, the importance of a chemically clean substrate to provide the best possible contact surface for the applied PTFE coating cannot be over-emphasised.
Residues of oil, grease, marking inks, cutting oils etc. after manufacturing operations will seriously affect the adhesion of applied coatings and must be removed. It is erroneous to think that subsequent cleaning operations will remove such contaminants and it is bad practice to permit them to remain on the surface. Failure to remove these contaminants before blast cleaning results in them being distributed over the steel surface and contaminating the abrasive. Suitable organic solvents, emulsion degreasing agents or equivalents should be applied to remove contaminants in preparation for subsequent descaling treatments.
The type and size of the abrasive used in blast cleaning have a significant effect on the profile or amplitude produced. In addition to the degree of cleanliness, surface preparation specifications need to consider ‘roughness’ relative to the coating to be applied. For example, shot abrasives are used for thin film paint coatings such as pre-fabrication primers, whereas thick or high build paint coatings and thermally sprayed metal coatings need a coarse angular surface profile to provide a mechanical key. Inadequate quality control and lack of restriction of large abrasive particle sizes for thin priming coats can lead to peaks of the blast cleaned surface not being adequately covered and may produce rust spots very quickly.
The blast cleaning operation produces large quantities of dust and debris which must be removed from the abraded surface. Automatic plants are usually equipped with mechanical brushes and air blowers. Other methods can utilise sweeping and vacuum cleaning. However, the effectiveness of these cleaning operations may not be readily visible and the presence of fine residual dust particles that could interfere with coating adhesion.
Dry abrasive blast cleaning can achieve a high standard of surface cleanliness using versatile blast cleaning equipment at relatively low cost. By means of specially adapted equipment, it is now possible to enclose and recover abrasives and particulate matter within an enclosed vacuum system. The equipment enables a fairly dust-free surface to be produced and the recycling of the abrasive.